Evidence has been presented in the recent past that some aspects of allograft tolerance can be explained by the presence of serum factors blocking the specific lymphocytes that react against cells carrying "tolerant" antigens. The purpose of the present study is to: a) analyze the changes in immune response during the course of induction of neonatal tolerance in rats; b) analyze the changes in the immune response during and after termination of the tolerant state; c) correlate the in vitro results with the in vivo status of a tolerated allograft and d) induce specific non-reactivity to allograft (kidney) antigens in dogs (to circumvent the danger of immunosuppression therapy) and analyze the role of serum factors in such dogs. The future application of such studies lies in revealing the mechanism of tolerance and its possible relationship to immunological enhancement (in both organ transplants and tumor rejection).